Recently, Homogeneous-Charge Compression Ignition (HCCI) combustion in which gasoline fuel mixed with air is combusted by self-ignition inside a sufficiently compressed combustion chamber has attracted attention. The HCCI combustion is a mode in which the mixture gas combusts at a plurality of positions simultaneously, and thus has a faster combustion speed of the mixture gas than in SI combustion (spark-ignition combustion) which is adopted for general gasoline engines. Therefore, the HCCI combustion is said to be significantly advantageous in terms of thermal efficiency. However, the HCCI combustion has issues such as a combustion start timing of the mixture gas (a timing at which the mixture gas self-ignites) greatly varying due external factors (e.g., atmospheric temperature) and a control during a transient operation in which an engine load sharply changes being difficult.
Therefore, instead of combusting all of the mixture gas by self-ignition, it is proposed to combust a portion of the mixture gas by spark-ignition using a spark plug. That is, after forcibly combusting a portion of the mixture gas through flame propagation caused by spark-ignition (SI combustion), the remaining mixture gas is combusted by self-ignition (CI combustion). Hereinafter, such combustion is referred to as “partial compression-ignition combustion.”
For example, JP2009-108778A discloses an engine adopting a similar concept to the partial compression-ignition combustion. This engine causes flame propagation combustion by spark-igniting a stratified mixture gas which is formed around a spark plug by a supplementary fuel injection, and then performs a main fuel injection inside a combustion chamber warmed up by an effect of the flame propagation combustion, so as to combust through self-ignition the fuel injected in the main fuel injection.
Although the engine of JP2009-108778A can stimulate CI combustion by the spark-ignition using the spark plug, a state of a flame core formed immediately after the spark-ignition is considered to vary to some extent due to an environment of the combustion chamber. For example, when an engine load is low, since an injection amount of the fuel is small which easily makes an air-fuel ratio of the mixture gas lean, even if the spark-ignition is performed, the flame core may not grow sufficiently. When the growth of the flame core is insufficient, a start timing of the CI combustion may greatly deviate from a target timing and the combustion may become unstable.